Short-term radon test kits are not accurate enough for big decisions about radon mitigation. Repeat the tests or use a long-term test kit to provide a better picture of the real radon level in your home.
Radon Levels Fluctuate Wildly
During a single day, the concentration of radon gas in indoor air varies widely and may easily double or triple. Moreover, it fluctuates greatly from day to day, week to week, and season to season.
Radon gas is drawn from the ground into homes by differences in concentration, air pressure, and temperature. This force largely depends on the weather and ground conditions outdoors. The indoor radon level is thus affected by barometric pressure, strong winds, rain-soaked ground, snow cover, the season, heating and A/C systems, house construction, open windows, etc.
Short-term radon detectors (2 to 7 days) provide just a snapshot of a widely fluctuating value. They do not provide an accurate estimate of the average radon concentration in the home.
Radon Test Results vs. Reality
The chart (click to enlarge!) compares short-term and long-term radon test results in a house during a period of two years (courtesy of St. John’s University, MN).
The house has a somewhat elevated radon level above the U.S. EPA’s “Action Limit” of 4 pCi/L as revealed by the two year-long measurements. It should be repaired.
About 40% of the short-term tests incorrectly indicate that the radon level is below the Action Limit (“false negatives”). Although the home should be mitigated, the homeowner may feel confident that there is no radon problem.
Even the 90-day radon tests show false negatives in two out of eight tests (or 25%).
Shorter test results may be misleading. The shorter the test, the wider the range of test results:
Range of Test Results (pCi/L)
| Length of Test | Low value | High value |
| year-long | 4.5 | 5.0 |
| 90 days | 3.0 | 7.6 |
| 7 days | 1 | 11 |
| 2 days | 1 | 13 |
| hourly | 0 | 16 |
What is the Safe Level of Radon?
Radon is a radioactive gas – “unsafe at any level” (per the U.S. EPA). The risk to your health increases proportionally to the radon level. Women and children are more vulnerable.
EPA recommends that you should “fix your home” if the radon level is above 4 pCi/L (the Action Level) and “consider fixing your home” it the radon level is above 2 pCi/L.
The World Health Organization sets the recommended radon reference level for residential structures at 2.7 pCi/L (100 Bq/m3).
How Accurate Are Short-Term Home Radon Tests?
EPA recognizes the temporal variation of radon levels and cautions that short-term measurements may not be accurate (The Citizen’s Guide). If the result is at or above 4 pCi/L, EPA recommends another short-term test. However, studies show that two short-term radon measurements are only marginally better than a single test.
Another confounding issue is the accuracy of short-term radon test kits. EPA’s guidelines on accuracy set individual relative errors at below 25% and precision at a coefficient of variation below 10% at 4 pCi /L. But studies have shown that most radon kits do not meet these guidelines.
Radon tests can have two types of error:
- “False positive” – the test result is above 4 pCi/L although the long-term average is below it. The homeowner may “waste money” on a radon mitigation system. However, EPA recommends that homeowners consider fixing their home if the radon test is above 2 pCi/L and explains that radon gas is unsafe at any level. Therefore, reducing the radon level is not a waste.
- “False negative” – the test result is below the Action Level but the long-term radon level is above it. The homeowner may decide not to mitigate although he should. This will increase the health risk to the entire family. Therefore, homeowners should follow EPA’s other recommendation to “consider” reducing radon if the test result is above 2 pCi/L.
Short-term radon tests do not provide an accurate estimate of the annual average concentration and are not a sound basis for radon mitigation decisions.
However, as a matter of public health policy, short-term radon tests are valuable for raising public awareness and for screening of the housing stock. They may not provide a clear answer to the individual homeowner but will save lives by encouraging homeowners to reduce the radon concentration in their homes.
Long-Term Radon Test Kits
The long-term test kits are alpha track detectors that record the radioactive breakup of each radon atom. Long-term tests are performed over a period of 3 to 12 months which averages out the wild short-term variations.
If you do not need the test results quickly, the long-term test will provide a much more accurate result.
The long-term test results are often by more than a factor of 2 different from short-term results and sometimes by more than a factor of 4.
Real Estate Sales
Considering the wide fluctuations in radon level, the difference between 3.5 and 4.5 pCi/L short-term results is meaningless. The long-term radon level may easily be a double or a half of that. Then a short-term test result of around 4 pCi/L means that the actual long-term radon level is very likely in the 2 to 8 pCi/L range.
When the short-term result is in the “gray area” of 2 – 8 pCi/L, it is a game of roulette for the home seller and the new homeowner.
If the short-term result is 4.1 pCi/L, the seller of a house may be forced to spend over $1,000 on radon mitigation although the actual long-term radon level may be below 2 pCi/L. Although this may be unfair to the seller, a reduction in radon concentration to below 2 pCi/L would certainly benefit future homeowners.
On the other hand, if the short-term test is 3.9 pCi/L, the actual long-term level might be double that. The potential buyer should get a long-term test kit, or better yet, work to reduce the radon level to below 2 pCi/L in order to provide a healthier home for his family.
What Should You Do?
Test the radon level in your home every year or at least every two years, as EPA recommends. The underground flows of radon gas change over time, particularly if there is construction nearby.
If the short-term radon test result is below 2 pCi/L, it is almost certain that your home meets the EPA Action Limit.
If the short-term test is above 8 pCi/L, it is very likely that it exceeds the Action Level and you need radon mitigation.
If the radon test result is in the gray area 2 to 8 pCi/L, you have several options:
- another short-term test, preferably in another season, but this may be completely useless
- a long-term radon test over 3 or preferably 6 months which will give you the true picture
- reduce the radon level to a practical minimum
If you are concerned that you may be “wasting money” on radon reduction while your actual radon level is below 4 pCi/L, don’t. Radon is unsafe at any level and you should always try to it to a minimum. It is for your family. If the basement is still unfinished and unpainted, it is an easy do-it-yourself repair by sealing the basement with RadonSeal concrete sealer. Or contact a licensed radon mitigation contractor to install a sub-slab depressurization system that will draw the gas from underneath the basement slab with a fan.
One day, you may want to put your house on the market. Avoid the last-minute problems with radon and play it safe. Reduce the radon level now to below 2 pCi/L! Check the radon level with a long-term test kit.